This invention relates to monitoring of combustion in a cylinder of a reciprocating piston internal combustion engine.
A primary purpose of monitoring combustion within a cylinder of a reciprocating piston internal combustion engine is to ascertain the crank-shaft position at which combustion of fuel within the cylinder commences. The point in the operation of the engine at which combustion commences is an important parameter in the control of engine operation by means of a closed- or open-, loop feedback control system of an engine, and U.S. Pat. specification No. 4,381,748 discloses engine control systems using the start of combustion as a main control parameter. The start of combustion is not the only parameter which may be used by an engine control or engine management system and other composition related control parameters include the pressure within the cylinder and the combustion temperature.
It is known, at least from U.S. Pat. No. 4,381,748, to monitor the combustion within an engine cylinder by optical means. In U.S. Pat. No. 4,381,748 a phototransistor is disposed at the exterior of a transparent window in the wall of the cylinder and monitors light intensity within the cylinder. Such an arrangement however has significant disadvantages. Firstly, it is recognized that the peak light intensity will vary from cycle to cycle of the engine, and thus in order to determine satisfactorily the point at which a signal representative of for example the start of combustion is passed to the control system it is necessary to establish, over a predetermined number of cycles, an average peak intensity, and then to pass a signal to the control system when the light intensity perceived by the photo-transistor achieves a predetermined percentage of the average peak intensity. The time constant which occurs in the system in order to permit calculation of an average peak intensity slows down the system response and thus the control system cannot respond rapidly to engine operating changes, and may not cope adequately with transient changes.
Naturally the window in the cylinder wall will become contaminated during operation of the engine and such contamination will alter the light intensity perceived by the photo transistor. Such changes in perceived intensity can give rise to the control system effecting non-optimum control of the engine operation.
British Patent No. 2,098,756 also discloses a system which monitors light intensity in a cylinder of an engine but differing from U.S. Pat. No. 4,381,748 in that the intensity of a plurality of predetermined wavelengths is monitored. It is to be expected therefore that the system of British Patent No. 2,098,756 will suffer from the disadvantages mentioned above in relation to U.S. Pat. No. 4,381,748.